Hydrocarbons with at least five carbon atoms per molecule (hereinafter referred to as "C.sub.5.sup.+ hydrocarbons") can be prepared from hydrocarbons having at most four carbon atoms per molecule (hereinafter referred to as "C.sub.4.sup.- hydrocarbons") by a two-step process in which the C.sub.4.sup.- hydrocarbons are converted in the first step by steam reforming into a mixture of carbon monoxide and hydrogen, which mixture is subsequently converted in the second step into a mixture of hydrocarbons consisting substantially of C.sub.5.sup.+ hydrocarbons by contacting it at elevated temperature and pressure with a catalyst. The reaction which takes place in the second step of the process is known in the literature as the Fischer-Tropsch hydrocarbon synthesis. Catalysts often used for the purpose comprise one or more metals from the iron group, together with one or more promoters, and a carrier material. These catalysts can suitably be prepared by the known techniques, such as precipitation, impregnation, kneading and melting. The products which can be prepared by using these catalysts usually have a very wide range of molecular weight distribution and, in addition to branched and unbranched paraffins, often contain considerable amounts of olefins and oxygen-containing organic compounds. Usually only a minor portion of the products obtained is made up of middle distillates. Of these middle distillates not only the yield but also the pour point is unsatisfactory. Therefore, the direct conversion of H.sub.2 /CO mixtures according to Fischer-Tropsch is not a very attractive route for the production of middle distillates on a technical scale.
In this patent application "middle distillates" should be taken to be hydrocarbon mixtures whose boiling range corresponds substantially with that of the kerosene and gas oil fractions obtained in the conventional atmospheric distillation of crude mineral oil. The middle distillate range lies substantially between about 150.degree. and 360.degree. C.
Recently a class of Fischer-Tropsch catalysts was found which has the property of yielding a product in which only very minor amounts of olefins and oxygen-containing organic compounds occur and which consists virtually completely of unbranched paraffins, a considerable portion of which paraffins boils above the middle distillate range. It has been found that the high-boiling part of this product can be converted in high yield into middle distillates by hydrocracking. As feed for the hydrocracking, at least the part of the product is chosen whose initial boiling point lies above the final boiling point of the heaviest middle distillate desired as end product. The hydrocracking, which is characterized by a very low hydrogen consumption, leads to middle distillates with a considerably better pour point than those obtained in the direct conversion of a H.sub.2 /CO mixture according to Fischer-Tropsch.
The Fischer-Tropsch catalysts belonging to the above-mentioned class contain silica, alumina or silica-alumina as carrier material and cobalt together with zirconium, titanium and/or chromium as catalytically active metals, in such quantities that the catalysts comprise about 3-60 pbw of cobalt and about 0.1-100 pbw of zirconium, titanium and/or chromium per 100 pbw of carrier material. The catalysts are prepared by depositing the metals involved on the carrier material by kneading and/or impregnation. For further information on the preparation of these catalysts by kneading and/or impregnation, reference may be made to Netherlands Patent Application No. 8301922, which is commonly-assigned copending U.S. patent application, Ser. No. 594.618, filed Mar. 29, 1984, now U.S. Pat. No. 4,522,939, issued June 11, 1985.
Although the use of the afore-defined cobalt catalysts for the conversion of H.sub.2 /CO mixtures yields a product whose high-boiling part can be converted in a simple manner and in high yield into middle distillates, the use of these catalysts in the second step of the two-step process described hereinabove is attended with a number of drawbacks. As described hereinbefore the conversion of the C.sub.4.sup.- hydrocarbons in the first step can be carried out by steam reforming. With a view to the reactions which occur during the conversion of the C.sub.4.sup.- hydrocarbons and in order to minimize carbonization of the catalyst used, this conversion should be carried out by using a steam/hydrocarbon ratio higher than about 1 g mol/g atom C. The drawbacks attached to the use of the present cobalt catalysts in the second step combined with a conversion of the C.sub.4.sup.- hydrocarbons by steam reforming in the first step are connected with the fact that this conversion of the C.sub.4.sup.- hydrocarbons yields a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio which is considerably higher than about 2. This can be suitably demonstrated with the aid of the development of the reaction when methane is used as feed. In the steam reforming of methane two reactions occur, viz. a main reaction CH.sub.4 +H.sub.2 O.fwdarw.CO+3H.sub.2 and a side reaction CO+H.sub.2 O.fwdarw.CO.sub. 2 +H.sub.2. This reaction development, when H.sub.2 O/CH.sub.4 molar ratios between about 2 and about 3 which are generally used in practice are applied, leads to H.sub.2 /CO mixtures having H.sub.2 /CO molar ratios between about 3.8 and about 4.5. It should be noted here that with the present cobalt catalysts the H.sub.2 +CO conversion is smaller according as the H.sub.2 +CO molar ratio of the H.sub.2 /CO mixture supplied varies more from 2, and that in addition their C.sub.5.sup.+ selectivity is lower according as the H.sub.2 /CO mixture supplied has a higher H.sub.2 /CO molar ratio. Consequently, when the present cobalt catalysts are used for the conversion of a H.sub.2 /CO mixture prepared by steam reforming, this leads to both a low H.sub.2 +CO conversion and a low C.sub.5.sup.+ selectivity. Therefore, in view of the high H.sub.2 /CO molar ratio of the H.sub.2 /CO mixture formed therein, steam reforming is not very suitable to be used for the preparation of the feed for the present cobalt catalyst.
H.sub.2 /CO mixtures having considerably lower H.sub.2 /CO molar ratios than when steam reforming is used can be prepared by starting from C.sub.4.sup.- hydrocarbons and carrying out the conversion in the presence of carbon dioxide. This so-called carbon dioxide reforming, which, for the same reasons as given for the steam reforming, should be carried out by using a carbon dioxide/hydrocarbon ratio higher than about 1 g mol/g atom C. yields a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio which is considerably lower than about 2. As in the case with the steam reforming, this can suitably be demonstrated with the aid of the development of the reaction when methane is used as feed. In the carbon dioxide reforming of methane two reactions occur, viz. a main reaction CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 and a side reaction H.sub.2 +CO.sub.2 .fwdarw.H.sub.2 O+CO. This reaction development, when CO.sub.2 /CH.sub.4 molar ratios between about 1.5 and about 2 which are generally used in practice are applied, leads to H.sub.2 /CO mixtures having H.sub.2 /CO molar ratios between about 0.70 and about 0.64. Although replacing steam reforming with carbon dioxide reforming in the first step of the two-step process wherein the present cobalt catalysts are used in the second step offers a solution to the low C.sub.5.sup.+ selectivity problem (since the cobalt catalysts show a higher C.sub.5.sup.+ selectivity according as the H.sub.2 /CO molar ratio of the feed is lower), said replacement has no influence on the low H.sub.2 +CO conversion problem (caused by a H.sub.2 /CO molar ratio of the feed which varies considerably from 2) and gives rise to another problem. To solve these two problems a solution has now been found. High conversions of low-hydrogen H.sub.2 /CO mixtures using the present cobalt catalysts can be achieved by using these catalysts in a catalyst combination which has CO-shift activity. The fresh problem mentioned above is connected with the need of carrying out the two steps of the two-step process at substantially the same pressure in order to obviate compression of large gas volumes. Since the hydrocarbon synthesis over the cobalt catalyst of the second step requires a pressure higher than 10 bar, a corresponding high pressure must also be used in the first step. However, a drawback of the carbon dioxide reforming carried out at a high pressure is its low conversion. For instance, the carbon dioxide reforming of methane at a pressure of about 20 bar and a CO.sub.2 /CH.sub.4 molar ratio of about 2 yields a H.sub.2 /CO mixture having a H.sub.2 /CO molar ratio of about 0.64 at a methane conversion of not more than about 50%. This drawback can be taken away by carrying out the reforming in the presence of both carbon dioxide and steam. For instance, the above-described reforming of methane in the presence of carbon dioxide at a pressure of about 20 bar and a CO.sub.2 /CH.sub.4 molar ratio of about 2 in the presence of a quantity of steam corresponding with a H.sub.2 O/CO molar ratio of about 0.25 leads to an increase in methane conversion of from about 50 to about 91%, while the H.sub.2 /CO molar ratio of the H.sub.2 /CO mixture produced only increases from about 0.64 to about 0.66. More generally, it has been found that H.sub.2 /CO mixtures whose H.sub.2 /CO molar ratios may lie between about 0.25 and about 2.25 at choice can be prepared in high yield by reforming C.sub.4.sup.- hydrocarbons at a pressure higher than about 10 bar in the presence of carbon dioxide and steam, provided that the following requirements are met
(1) the carbon dioxide/hydrocarbon ratio (a) should be higher than about 0.2 but lower than about 10 g mol CO.sub.2 /g atom C,
(2) the steam/hydrogen ratio (b) should be higher than about 0.1 but lower than about 1 g mol H.sub.2 O/g atom C, and
(3) the carbon dioxide/steam ratio should be chosen such as to meet the requirement (2.times.a+3.times.b)&gt;3.
By using the afore-described reforming as the first step of the two-step process for the preparation of C.sub.5.sup.+ hydrocarbons from C.sub.4.sup.- hydrocarbons while using the present cobalt catalysts in the second step and at a pressure which corresponds substantially with that used in the first step, C.sub.5.sup.+ hydrocarbons can be prepared according to this two-step treatment in high yield and with high selectivity, provided that if the H.sub.2 /CO mixture prepared in the first step has a H.sub.2 /CO molar ratio lower than about 1.5, the cobalt catalyst should be used in a catalyst combination which has CO-shift activity.